Optical endpoint detection for buff module on CMP tool

ABSTRACT

The invention includes a polishing station and a buffing station for chemically mechanically polishing a wafer. The buffing station preferably includes a pair of rotating opposing buffing pads that receive a portion of the wafer. In the buffing station, a measurement instrument may be positioned adjacent an exposed portion of the wafer for detecting an endpoint of the buffing process. A first slurry may be used in the polishing station and a second slurry may be used in the buffing station. For planarizing a wafer with a metal layer over a barrier layer, the metal layer may be removed at the polishing station while the barrier layer is removed at the buffing station. For planarizing a dielectric layer, the top portion of the dielectric layer may be removed at the polishing station with an additional amount removed at the buffing station thereby leaving a dielectric layer with a desired thickness.

TECHNICAL FIELD

[0001] The invention relates to semiconductor manufacturing and more specifically to a method and apparatus for chemically mechanically polishing and buffing a wafer.

BACKGROUND OF THE INVENTION

[0002] A flat disk or “wafer” of single crystal silicon is the basic substrate material in the semiconductor industry for the manufacture of integrated circuits. Semiconductor wafers are typically created by growing an elongated cylinder or boule of single crystal silicon and then slicing individual wafers from the cylinder. The slicing causes both faces of the wafer to be extremely rough. The front face of the wafer on which integrated circuitry is to be constructed must be extremely flat in order to facilitate reliable semiconductor junctions with subsequent layers of material applied to the wafer. Also, the material layers (deposited thin film layers usually made of metals for conductors or oxides for insulators) applied to the wafer while building interconnects for the integrated circuitry must also be made a uniform thickness.

[0003] Planarization is the process of removing projections and other imperfections to create a flat planar surface, both locally and globally, and/or the removal of material to create a uniform thickness for a deposited thin film layer on a wafer. Semiconductor wafers are planarized or polished to achieve a smooth, flat finish before performing process steps that create the integrated circuitry or interconnects on the wafer. A considerable amount of effort in the manufacturing of modern complex, high density multilevel interconnects is devoted to the planarization of the individual layers of the interconnect structure. Nonplanar surfaces create poor optical resolution of subsequent photolithography processing steps. Poor optical resolution prohibits the printing of high-density lines. Another problem with nonplanar surface topography is the step coverage of subsequent metalization layers. If a step height is too large there is a serious danger that open circuits will be created. Planar interconnect surface layers are required in the fabrication of modem high-density integrated circuits. To this end, chemical-mechanical polishing (CMP) tools have been developed to provide controlled planarization of both structured and unstructured wafers.

[0004] CMP consists of a chemical process and a mechanical process acting together, for example, to reduce height variations across a dielectric region, clear metal deposits in damascene processes or remove excess oxide in shallow trench isolation fabrication. The chemical-mechanical process is achieved with a liquid medium containing chemicals and abrasive particles (commonly referred to as slurry) that react with the front surface of the wafer while it is mechanically stressed during the planarization process.

[0005] In a conventional CMP tool for planarizing a wafer, a wafer is secured in a carrier connected to a shaft. Pressure is exerted on the back surface of the wafer by the carrier in order to press the front surface of the wafer against the polishing pad in the presence of slurry. The wafer and/or polishing pad are then moved in relation to each other via motor(s) connected to the shaft and/or platen in order to remove material in a planar manner from the front surface of the wafer. Various combinations of motions are known for moving the wafer and polishing pad in relation to each other. For example, the wafer is commonly rotated or held stationary and the a polishing pad is moved in either a linear, rotational or orbital manner. After planarizing the wafer on the polishing pad, the wafer may be buffed at a buff station. The buffing is typically used to remove microscratches on the front surface of the wafer and to perform a rough cleaning of the wafer. Applicants have noticed that in certain cases it would be desirable to remove a specific amount of a dielectric, metal film, barrier layer or opaque film from the front surface of the wafer during the buffing of the wafer. However, it is difficult to remove the desired amount of material in the buff station with the conventional approach of using a predetermined set process time. The wafer is then transported to a cleaning station to be cleaned, rinsed and dried.

[0006] One conventional method of polishing a thin film of tungsten on a CMP tool involves removing the bulk of the tungsten overburden with an optimized, low with-in-wafer-nonuniformity (WIWNU) process on an orbital polishing station. As the bulk tungsten clears, a change in carrier current is detected and the transition is made to an overpolish step. During the overpolish step, the remaining tungsten is removed along with the barrier layers (typically TiN and Ti) and some oxide. The overpolish step is performed on the same pad and with the same slurry as the bulk tungsten removal step. Applicants have noticed that a certain amount of tungsten erosion in the plugs and/or line areas occurs due to the chemically reactive nature of the tungsten slurry. A buff station is then used to remove microscratches and other surface defects induced by the CMP process, but the erosion in the plugs and/or line areas remain.

[0007] What is needed is a method and apparatus for uniformly planarizing a wafer that avoids the problems of the prior art. The method and apparatus needs to provide a simple solution for accurately determining endpoint while improving the throughput of the CMP tool. For metal planarization, the solution needs to minimize erosion in the plugs and/or line areas. For dielectric planarization, the solution needs to remove microscratches while leaving a desired thickness of the dielectric layer.

SUMMARY OF THE INVENTION

[0008] The present invention removes material from a front surface of a wafer in a substantially planar and uniform manner. The invention may advantageously be part of a chemical mechanical polishing tool comprising a polishing station and a buff station. The wafer to be planarized may have one or more deposited thin films of metal or a dielectric or be an STI or unprocessed wafer.

[0009] The polishing station preferably includes an orbital motion generator for orbiting a polishing pad. For some embodiments of the invention, other motions, e.g. linear or rotational, may be implemented for the polishing pad. A carrier may be used to press the wafer against the polishing pad in order to remove material from the front surface of the wafer. The polishing station needs to be able to remove a desired depth of material from the wafer in a planar and uniform manner. The polishing station may have an endpoint detection system to enable the polishing station to terminate the polishing of the wafer after the desired depth of material has been removed.

[0010] The buff station includes a dual opposing buff pad system and an endpoint detection system. The dual opposing buff pad system includes a pair of adjacent buffing pads connected to one or more motors for rotating the pads. The buffs pads are preferably disk shaped and smaller in diameter than the wafer. The wafer may be placed between the buffing pads and supported by stanchions. The wafer so situated will rotate freely as the buff pads are rotated. Many endpoint detection systems are known in the art and may be used to practice the invention. The preferred endpoint detection system is a multifrequency optical system that uses a spectrometer, but a monochromatic optical system that uses a laser interferometer may also be used. The endpoint detection system also includes a probe. Because the buff pads are preferably smaller than the wafer, the probe may easily be positioned to measure exposed portions of the front surface of the wafer not covered by the buff pads. After buffing, the wafer is preferably cleaned and dried in a cleaning station.

[0011] The wafer may have a deposited barrier and metal layer on its front surface. This wafer may be planarized by removing substantially the entire metal layer on a polishing station followed by removing substantially the entire barrier layer on a buffing station. A first slurry that is chemically reactive with the metal layer may be used at the polishing station while a second slurry that is less chemically reactive with the metal layer may be used at the buffing station.

[0012] The wafer may have a dielectric layer deposited on its front surface. This wafer may be planarized by removing a desired thickness of the dielectric layer on a polishing station followed by removing microscratches caused by the polishing station on a buffing station. A first slurry that is chemically reactive with the dielectric layer may be used at the polishing station while a second slurry that is less chemically reactive with the metal layer may be used at the buffing station.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The present invention will hereinafter be described in conjunction with the appended drawing figures, wherein like numerals denote like elements, and:

[0014]FIG. 1A is a magnified side view of a portion of a wafer after a metal deposition process;

[0015]FIG. 1B is FIG. 1A after the overburden metal has been chemically mechanically polished away;

[0016]FIG. 1C is FIG. 1B after the barrier layer has been buffed way;

[0017]FIG. 1D is a magnified side view of a portion of a wafer that has experienced undesirably erosion and dishing.

[0018]FIG. 1E is a magnified side view of a portion of a wafer after a dielectric deposition process;

[0019]FIG. 1F is FIG. 1E after a top portion of the dielectric layer has been chemically mechanically polished away;

[0020]FIG. 1G is FIG. 1F after the remaining dielectric layer has been buffed to remove microscratches;

[0021]FIG. 2 is a top view of a buffing station;

[0022]FIG. 3 is a side view of a buffing station;

[0023]FIG. 4 is a top view of a chemical mechanical polishing tool with integrated buffing, cleaning and drying capabilities;

[0024]FIG. 5 is a side view of a mechanism for producing orbital motion for the chemical mechanical polishing station;

[0025]FIG. 6 is a flowchart for planarizing a metal film with a barrier layer; and FIG. 7 is a flowchart for planarizing a dielectric layer.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0026] An improved method and apparatus utilized in the polishing of semiconductor substrates and thin films formed thereon will now be described. In the following description, numerous specific details are set forth illustrating Applicant's best mode for practicing the present invention and enabling one of ordinary skill in the art to make and use the present invention. It will be obvious, however, to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known machines and process steps have not been described in particular detail in order to avoid unnecessarily obscuring the present invention.

[0027] The invention may be practiced with a chemical mechanical polishing (CMP) tool as shown in FIG. 4. The general design of the CMP tool shown in FIG. 4 is similar to a 776 model sold by SpeedFam-IPEC which is headquartered in Chandler, Ariz. One or more cassettes 400 loaded with wafers (not shown) may be loaded onto the CMP tool 420. A first robot 401 may slide along a track 401 as it removes wafers from the cassettes 400 and loads them into a holding station 403. A second robot 404 may take the wafer from the holding station 403 and place the wafer in one of two positions in a wet bath 403. A third robot 406 may remove the wafer from the wet bath 403 and transport the wafer to a carrier (not shown) associated with one of the four polishing stations 407. The carrier presses the wafer against the polishing surface of the polishing station 407 as relative motion is created between the front surface of the wafer and the polishing surface. The relative motion is preferably created by holding the front surface of the wafer in the carrier stationary while the polishing pad is orbited.

[0028] The polishing pad 104 may be orbited during the planarization process of the wafer 201 and rotated clockwise and counter-clockwise. FIG. 5 is a cross-sectional view of an exemplary motion generator 500 that may be used to generate an orbital motion for the platen 107 and polishing pad 104. The motion generator 500 is generally disclosed in U.S. Pat. No. 5,554,064 Breivogel et al. and is hereby incorporated by reference. Supporting base 220 may have a rigid frame 502 that can be securely fixed to the ground. Stationary frame 502 is used to support and balance motion generator 500. The outside ring 504 of a lower bearing 506 is rigidly fixed by clamps to stationary frame 502. Stationary frame 502 prevents outside ring 504 of lower bearing 506 from rotating. Wave generator 508 formed of a circular, hollow rigid body, preferably made of stainless steal, is clamped to the inside ring 510 of lower bearing 506. Wave generator 508 is also clamped to outside ring 512 of an upper bearing 514. Wave generator 508 positions upper bearing 514 parallel to lower bearing 506. Wave generator 508 offsets the center axis 515 of upper bearing 514 from the center axis 517 of lower bearing 506. A circular platen 211, preferably made of aluminum, is symmetrically positioned and securely fastened to the inner ring 519 of upper bearing 514. A polishing pad or pad assembly can be securely fastened to ridge 525 formed around the outside edge of the upper surface of platen 211. A universal joint 518 having two pivot points 520 a and 520 b is securely fastened to stationary frame 502 and to the bottom surface of platen 211. The lower portion of wave generator 508 is rigidly connected to a hollow and cylindrical drive spool 522 that in turn is connected to a hollow and cylindrical drive pulley 523. Drive pulley 523 is coupled by a belt 524 to a motor 526. Motor 526 may be a variable speed, three phase, two horsepower AC motor.

[0029] The orbital motion of platen 211 is generated by spinning wave generator 508. Wave generator 508 is rotated by variable speed motor 526. As wave generator 508 rotates, the center axis 515 of upper bearing 514 orbits about the center axis 517 of lower bearing 506. The radius of the orbit of the upper bearing 517 is equal to the offset (R) 526 between the center axis 515 of upper bearing 514 and the center axis 517 of the lower bearing 506. Upper bearing 514 orbits about the center axis 517 of lower bearing 506 at a rate equal to the rotation of wave generator 508. It is to be noted that the outer ring 512 of upper bearing 514 not only orbits but also rotates (spins) as wave generator 508 rotates. The function of universal joint 518 is to prevent torque from rotating or spinning platen 211. The dual pivot points 520 a and 520 b of universal joint 518 allow the platen 211 to move in all directions except a rotational direction. By connecting platen 211 to the inner ring 519 of upper bearing 514 and by connecting universal joint 518 to platen 211 and stationary frame 502 the rotational movement of inner ring 519 and platen 211 is prevented and platen 211 only orbits as desired. The orbit rate of platen 211 is equal to the rotation rate of wave generator 508 and the orbit radius of platen 211 is equal to the offset of the center 515 of upper bearing 514 from the center 517 of lower bearing 506.

[0030] It is to be appreciated that a variety of other well-known means may be employed to facilitate the orbital motion of the platen 211. While a particular method for producing an orbital motion has been given in detail, the present invention may be practiced using a variety of techniques for orbiting the platen 211. The platen 211 is preferably orbited with a radius between about 20 mm and 5 mm. It should also be understood that while an orbital motion for the platen 211 and polishing pad is preferred, the polishing pad may also be moved in other ways, e.g. linearly or rotationally.

[0031] With reference back to FIG. 4, a slurry may be introduced between the wafer and the polishing pad, preferably by pumping the slurry through the polishing pad directly to the wafer-polishing pad interface. A slurry that is reactive to the material being removed from the front surface of the wafer may be used to increase the removal rate of the material. A typical slurry may be SSW2000 for tungsten or SS12 for oxide; both manufactured by Cabot Microelectronics, headquartered in Aurora, Illinois. The third robot 406 may be used to take the wafer from the carrier in one of the polishing station 407 and transport the wafer to one of two buff stations 408.

[0032]FIGS. 2 and 3 illustrate a simplified view of a buffing station 408. The wafer 100 is inserted between buff pads 200 a and 200 b. The buff pads 200 a and 200 b may be made of polyurethane, have a textured surface and have a diameter of about 130 mm for a 200 mm wafer. The buff pad 200 a and 200 b may be, for example, a Politex Supreme model manufactured by Rodel Inc., headquartered in Phoenix, Ariz. The buff pads 200 a and 200 b may be connected to one or more corresponding motors 203 a and 203 b via shafts 201 a and 201 b. The motors 203 a and 203 b preferably drive the buff pads 200 a and 200 b counterclockwise at 300 rpms as shown by arrows A2 and A3. The rotation of the buff pads 200 a and 200 b will rotate the wafer 100 supported by freely rotating stanchions 202 as shown by arrow A1. Slurry 208 from a holding tank 207 may be used to wet the buffing pads 200 a and 200 b and improve the buffing process. The slurry 208 may advantageously be selected to be reactive with areas where material should be removed and less reactive with areas where material should not be removed. The slurry 208 will generally not be as reactive with the material on the front surface of the wafer as the slurry from the polishing station. Examples of slurries that may be used are SSW2000 or deionized water.

[0033] An optical probe 206 may be placed close to the front surface of the wafer 100. The layout of the buffing station 408 as described allows the probe 206 easy access to portions of the wafer 100 not covered by the buff pads 200 a and 200 b. A measuring instrument 204, such as a spectrometer or interferometer, may receive reflected light from the front surface of the wafer 100 and probe 206 via a fiber optic cable 205. The probe 206, fiber optic cable 205 and measurement instrument 204 may be part of, or replaced by, an endpoint detection system. For example, a Sentinal optical endpoint system manufactured by SpeedFam-IPEC, headquartered in Chandler, Ariz. may be used to detect endpoint on ILD, STI, tungsten and copper applications. Applicants have discovered that due to the slow removal rate and small amount of material removed, extremely accurate readings may be made in real time. The accurate determination of the endpoint greatly reduces dishing and erosion of metal lines and greatly enhances the accuracy of the thickness of a dielectric layer. The endpoint system for tungsten vias and dual damascene polishing would reduce defectivity and improve dishing and erosion and interlayer dielectric (ILD) would benefit by a controlled final wafer oxide thickness.

[0034] Referring back to FIG. 4, while the wafer was buffed in one of the buff stations 408, the polishing pad in one or more of the polishing stations may be conditioned by a polishing pad conditioner 409 sweeping across the surface of the polishing pad. After the wafer has been buffed at one of the buffing stations 408, the wafer may be transported by the third robot 406 back to one of the wet baths 405. The second robot 404 may then remove the wafer from the wet bath 405 and transport the wafer to a first cleaning position 410 within a cleaning station 414. After an initial cleaning in the first cleaning position 410, a fourth robot 412 may transport the wafer to a second cleaning position 411. Cleaning positions 410 and 411 may be of types known in the art and may be, for example, similar in layout to the buffing stations 408 already described. If the cleaning positions 410 and 411 are similar to the buff stations 408, softer pads and cleaning solutions will be used. Alternatively, the cleaning positions 410 and 411 may comprise a plurality of pairs of opposing rollers aligned so that the wafer may be pulled through the center of them. After cleaning in cleaning positions 410 and 411, the fourth robot will move the wafer to a drying unit 413. The drying unit 413 is preferably a spin drier that dries the wafer by rapidly spinning the wafer and removing the fluids on the wafer by centrifugal force. The dried wafer may be removed from the cleaning station 414 by the first robot 401 and replaced into one of the cassettes 400.

[0035] A detailed layout of one possible CMP tool has thus been described. Of course, many variations in the CMP tool design with, for examples, a different number of robots, polishing station and/or buffing stations or a different layout may also be used.

[0036] A description of a process for removing a deposited metal layer from the top surface of a wafer will now be described with continuing references to FIGS. 1A, 1B, 1C, and 6. The wafer 100 in FIG. 1A has had grooves etched from a dielectric layer 103 and a thin barrier layer 102, e.g. Ti or TiN, is deposited over the grooves and dielectric layer 103. The barrier layer 102 prevents the metal in the metal lines 101 b from migrating into the dielectric layer 103 and possible causing current leaks between lines 110 b. A metal layer, e.g. aluminum, tungsten or copper, is deposited over the barrier layer 102. The metal layer thus comprises the desirable metal lines 101 b that have been deposited in the grooves and the undesirable metal overburden 110 a.

[0037] The metal overburden 110 a shown in FIG. 1A may be removed by chemical mechanical polishing of the wafer 100 at a polishing station leaving the barrier layer 102 and metal lines 110 b in the grooves as shown in FIG. 1B. The slurry used at the polishing station may advantageously be chosen to assist in the removal of the metal overburden 110 a. (Step 600) Care should be taken to terminate the CMP process before dishing or erosion 104 occurs as shown in FIG. 1D. Erosion 104 is a problem and may easily occur because the slurry for the polishing station will generally be reactive with the metal lines 101 b.

[0038] The wafer may be transported to a buffing station to remove the barrier layer 102. A slurry may be chosen for the buffing station that will assist in removing the barrier layer 102 while minimizing the removal of the metal lines 101 b as shown in FIG. 1C. (Step 601) After buffing, the wafer 100 may be cleaned and dried (Step 602).

[0039] A description of a process for leaving a desired thickness of a deposited dielectric layer on the top surface of a wafer will now be described with continuing references to FIGS. 1E, 1F, 1G, and 7. The wafer 100 in FIG. 1E has had a dielectric layer 103 a, typically an oxide, deposited onto the top surface of the wafer 100 after the previously described process of planarizing a metal layer on a wafer 100. The dielectric layer 103 a will be used to electrically separate the lines of wire 101 from one level from the lines of wire in another level.

[0040] A portion of the dielectric layer 103 b shown in FIG. 1F may be removed by chemical mechanical polishing of the wafer 100 at a polishing station leaving only slightly more than the final desired thickness of the dielectric layer 103 a. The slurry used at the polishing station may advantageously be chosen to assist in the efficient removal of the dielectric layer 103 b. (Step 700) However, slurries that assist in the efficient removal of the dielectric layer 103 b are also likely to cause microscratches and defectivities.

[0041] The wafer may be transported to a buffing station to remove a thin layer of the dielectric layer 103 c. By removing this thin layer 103 c, the microscratches and defectivities in the dielectric layer 103 a may also be removed. Another slurry may be chosen for the buffing station that will remove the microscratches and defectivities without causing new microscratches or defectivities as shown in FIG. 1G. (Step 701) This second less reactive slurry may be used in the buffing station because less material needs to be removed from the dielectric layer. After buffing, the wafer 100 may be cleaned and dried (Step 702).

[0042] By removing the barrier layer 102 for metal planarization or microscratches and defectivities for dielectric planarization at the buffing station, Applicants have discovered several advantageous. One advantage is that a first slurry may be used on the polishing station that is more reactive while a second slurry may be used on the buffing station that is less reactive. This allows greater control over the important final phase of material removal. By separating the point of use for the different slurries, the plumbing is simplified and minimal mixing of the slurries occurs as typically happens when both slurries are used at the same station. Another advantage is that greater through-put may be achieved by many CMP tools where the other stations in the CMP tool wait on the polishing stations, i.e. the polishing stations are a bottle-neck. The increase in throughput occurs since the buffing stations take over some of the workload from the polishing stations. Another advantage is that a more accurate termination of the planarization process, i.e. endpoint, may be determined at the buffing stations instead of at the polishing stations. The material removal rate is much slower at the buffing stations making it easier to determine endpoint in real time. In addition, when using a buffing station as previously described, much of the front surface of the wafer is exposed during the buffing process making it easier to take accurate measurements to determine endpoint.

[0043] While the invention has been described with regard to specific embodiments, those skilled in the art will recognize that changes can be made in form and detail without departing from the spirit and scope of the invention. 

We claim:
 1. A buffing station for removing material from a front surface of a wafer comprising: a) a pair of adjacent buff pads for receiving a first portion of the wafer; b) at least one motor for rotating the buff pads; and c) an end point detection system comprising a probe and a measurement instrument for taking measurements on the front surface of the wafer.
 2. The buffing station of claim 1 wherein the probe is positioned adjacent a second portion of the wafer that is different than the first portion of the wafer.
 3. The buffing station of claim 1 wherein the buff pads are disk shaped.
 4. The buffing station of claim 3 wherein the diameter of the buff pads is smaller than the diameter of the wafer.
 5. The buffing station of claim 1 wherein the measurement instrument comprises a spectrometer.
 6. The buffing station of claim 1 wherein the measurement instrument comprises an interferometer.
 7. An apparatus for removing material from a front surface of a wafer comprising: a) a polishing station; and b) a buff station, wherein the buff station comprises a dual opposing buff pad system and an endpoint detection system.
 8. The apparatus of claim 7, wherein the polishing station comprises an orbital motion generator and a polishing pad connected to the orbital motion generator.
 9. The apparatus of claim 7 further comprising: c) a cleaning station wherein the wafer is cleaned and dried.
 10. A method of planarizing a front surface of a wafer with a deposited barrier and metal layer comprising the steps of: a) removing substantially the entire metal layer on a polishing station; and b) removing substantially the entire barrier layer on a buffing station wherein the buffing station comprises a dual opposing buff pad system and an endpoint detection system.
 11. The method of claim 10 further comprising the steps of: c) delivering a first slurry to the polishing station that that has a first level of chemically reactivity with the metal layer; and d) delivering a second slurry to the buffing station that has a second level of chemically reactivity with the metal layer, wherein the first level of chemical reactivity is greater than the second level of chemical reactivity.
 12. The method of claim 10 further comprising the step of: c) orbiting a polishing pad during the removal of the metal layer on the polishing station.
 13. The method of claim 10 further comprising the steps of: c) cleaning the wafer in a cleaning station; and d) drying the wafer in the cleaning station.
 14. A method of planarizing a front surface of a wafer with a deposited dielectric layer comprising the steps of: a) removing a desired thickness of the dielectric layer on a polishing station; and b) removing microscratches caused by the polishing station on a buffing station wherein the buffing station comprises a dual opposing buff pad system and an endpoint detection system.
 15. The method of claim 14 further comprising the steps of: c) delivering a first slurry to the polishing station that that has a first level of chemically reactivity with the dielectric layer; and d) delivering a second slurry to the buffing station that has a second level of chemically reactivity with the dielectric layer, wherein the first level of chemical reactivity is greater than the second level of chemical reactivity.
 16. The method of claim 14 further comprising the step of: c) orbiting a polishing pad during the removal of the metal layer on the polishing station.
 17. The method of claim 14 further comprising the steps of: c) cleaning the wafer in a cleaning station; and d) drying the wafer in the cleaning station. 